1. Field of the Invention
The present invention relates to a sheet feeder primarily suitable for use with a printer which feeds stacked sheet (plain sheet, coated sheet, sheet used for an OHP (Over-head Projector), glossy sheet, separated sheets such as glossy films, or envelopes) from the top one by one. Further, the present invention relates to a printer which produces a print by feeding sheets of sheet one by one, and more particularly, to the technique of feeding and conveying sheet.
2. Related Art
A feeder provided in a printer is generally known to be classified into a pawl-separation type and a pad-separation type.
As is publicly known, the pawl-separation type sheet feeder comprises pawls which are engaged with the front corners of stacked sheet and a sheet feed roller provided behind the pawls (i.e., in a downstream direction with reference to the direction in which the sheet is fed). The sheet feed roller is rotated so as to deflect the uppermost sheet of the sheet in a position between the sheet feed roller and the pawls. When the deflection of the sheet reaches a predetermined maximum value, the sheet is flipped to thereby separate the uppermost sheet from the next sheet. As a result, only the uppermost sheet is fed.
In contrast, as is also publicly known, the pad-separation type sheet feeder comprises a sheet feed roller and a separation pad. Taking a coefficient of friction between the sheet feed roller and sheet as .mu.1, a coefficient of friction between the separation pad and sheet as .mu.2, and a coefficient of friction between sheets of sheet as .mu.3, the sheet roller and the separation pad are arranged so as to satisfy the relationship of .mu.1&gt;.mu.2 &gt;.mu.3. Sheet is nipped between the rotatable sheet feed roller and the separation pad to be brought into pressed contact with the sheet feed roller, thereby separating the uppermost sheet from the next sheet. As a result, only the uppermost sheet is fed.
In either case, when the rotatable roller comes into contact with the surface of the uppermost sheet and the uppermost sheet is fed, the reverse side of the roller-contacted portion of the uppermost sheet makes slidable contact with the surface of the next sheet. The surface of the next sheet is slightly flawed by the sliding action of the uppermost sheet. If the sheet is, e.g., glossy sheet or a glossy film, the thus-produced flaws become noticeable.
For the case of the pawl-separation type sheet feeder, the portion of the uppermost sheet behind the pawls, i.e., the portion of the uppermost sheet to be deflected (in other words, the flawed protion), is positioned behind sufficient to be deflected, thereby impairing a print area of the sheet.
In contrast, for the case of the pad-separation type sheet feeder, although the front edge of the surface of only the sheet is flawed, the print area is not usually flawed.
Accordingly, the pad-separation type sheet feeder is superior from the viewpoint of the degree of flaw.
However, the pad-separation type sheet feeder has the following difficulties.
For example, as shown in FIG. 34, in a case where a pad-separation type sheet feeder 1 is applied to a printer, supplied sheet P1 is conveyed while being nipped between conveyor rollers 2, 3, and printing means 4 produces a print on the sheet P1. Usually, the sheet P1 still remains in the sheet feeder at a point in time when the conveyor rollers 2, 3 commence conveying the sheet P1.
In contrast, for the case of the pad-separation type sheet feeder, taking the frictional force acting between the sheet feed roller and sheet as f1. the frictional force acting between sheet and the separation pad as f2, and the the separation pad must be brought into pressed contact with each other in such a way as to satisfy the foregoing relationship of .mu.1&gt;.mu.2&gt;.mu.3, i.e., f1&gt;f2&gt;f3. If the sheet feed roller is held in pressed contact with the separation pad at a point in time when the conveyor rollers 2, 3 commence conveying the sheet P1, a rear portion of the sheet P1 is nipped between the sheet feed roller and the separation pad.
Consequently, the sheet P1 is conveyed by the conveyor rollers 2, 3 while it still remains under the load applied by the nipping section (or still being held in a rearwardly-withdrawn state or under a back-tension state) until the rear edge of the sheet P1 passes through the nipping section between the sheet feed roller and the separation pad.
If such a load (or a back tension) is great, the accuracy of sheet-feeding action of the conveyor rollers 2, 4 becomes lower, thereby resulting in a reduction in print quality. For this reason, it is desirable to reduce the load, or the nipping force exerted on the sheet feed roller and the separation pad, to load as small as possible. However, if the nipping force exerted on the sheet feed roller and the separation pad is too small, the next sheet gradually burrows into the nipping section between the sheet feed roller and the separation pad every time the sheet feed operation is repeated, thereby rendering the separation of sheet impossible.
A sheet medium alignment mechanism disclosed in Japanese Patent Application Laid-open No. Hei-7-53062 is intended to solve the foregoing problem.
FIGS. 35A to 35E show the construction and operation of this sheet medium alignment mechanism.
In these drawings, reference numeral 18 designates sheet feed rollers; 24 designates a separation pad; and 22 designates a lever.
As shown in FIG. 35C, the lever 22 is supported by a pivot A in a pivotable manner and is forced clockwise by a cantilever spring 26 which presses an arm 22c of the lever 22.
With this mechanism, as shown in FIGS. 35B to 35E, the uppermost sheet S1 is separated from the next sheet S2 by a nipping section formed between sheet feed rollers 18 and a separation pad 24 as a result of rotation of the sheet feed rollers 18 in a clockwise direction. As shown in FIG. 35D, only the uppermost sheet S1 is supplied. At this time, as shown in FIGS. 35C and 35D, the lever 22 is pressed to a receded position by the sheet S1. Even if the second sheet S2 (and a sheet S3 after the sheet S2) burrows into or attempts to burrow into the nipping section between the sheet feed rollers 18 and the separation pad 24, the lever 22 is pivoted clockwise by the urging force of the cantilever spring 26 immediately after the passage of the rear edge of the sheet S1 through the upper edge of the lever 22. As shown in FIG. 35E, the next sheet S2 (and the sheet S3 after the sheet S2) is forcibly reset to their original positions.
Accordingly, this mechanism seemingly solves the difficulty of separation of sheets owing to the gradual entry of the next sheet into the nipping section between the sheet feed rollers and the separation pad. However, this mechanism suffers from another problem which will be described later (see Problem 1).
If the sheet feed rollers 18 are rod-shaped, the roller is constantly held in pressed contact with the separation pad. Therefore, the sheet feed rollers and the separation pad are susceptible to abrasion.
There is a known sheet feeder which solves this problem, i.e., the abrasion of the sheet feed rollers and the separation pad, and comprises sheet feed rollers each of which has a substantially D-shaped lateral cross section.
FIG. 36 shows a sheet feeder disclosed in Japanese Utility Model Publication No. Hei-8-3396 as one example of the sheet feeder of this type. In the drawing, reference numeral 40 designates sheet feed rollers each of which has a substantially D-shaped lateral cross section and comprises a circular-arch portion 40a and a linear portion 40b.
Reference numeral 41 designates a guide block which supports a shaft 40c of the sheet feed rollers 40.
Reference numeral 42 designates a cassette and incorporates a sheet mounting plate 42a therein. A plurality of sheets of sheet P are loaded on the sheet mounting plate 42a in a stacked manner. Reference numeral 42c designates a spring which forces the sheet P toward the sheet roller 40.
Reference numeral 43 designates a separation pad and is attached to a bracket 48a. The separation pad 43 is on the course of rotation of the circular-arch portion 40a of the sheet feed rollers 40 and is pushed by a spring 44 along a guide 45 toward the sheet feed roller shaft 40c.
Reference numeral 46 designates an idle roller which is attached to the guide block 41 in a rotatable manner; and 47 designates a movable idle roller whose shaft 47a is fitted, in a rotatable manner, into an elongated groove 41a formed in the guide block 41. The movable idle roller 47 is forced toward the separation pad 43 by means of a spring 48 and is in contact with the separation pad 43.
Urging force F2 of the spring 48 is set to become smaller than urging force F1 of the spring 44 of the separation pad 43 (i.e., to satisfy a relationship of F1&gt;F2).
The sheet feeder having the foregoing construction operates in the following manner:
As shown in FIG. 36, when the sheet feeder is in a standby condition, the linear portion 40b of the sheet feed roller 40 is opposite to sheet P, and the sheet feed rollers 40 are kept from contact with the sheet P. Further, since the urging force F2 of the spring 48 of the movable idle roller 47 is set to become smaller than the urging force F1 of the spring 44 of the separation pad. 43, the movable idle roller 47 is pushed in an upward direction by the separation pad 43. As a result, the shaft 47a of the movable idle roller 7 is held in contact with the upper edge of the elongated groove 41a.
At the time of sheet feed operation, the sheet feed rollers 40 rotate in the direction designated by the arrow, and the circular-arch portion 40a comes into contact with the uppermost sheet P1 of the sheet P, whereby the sheet P1 is fed to the separation pad 43. At this time, the second sheet P2 is electrostatically attached to the sheet P1, or frictional force acts between the sheets P1 and P2, and the sheet P2 is sometimes fed together with the sheet P1.
However, the sheet P2 is separated from the sheet P1 by the separation pad 43 in the following manner, whereby only the uppermost sheet P1 is fed.
More specifically, the front edge of the sheet P2 collides with the separation pad 43 and is prevented from advancing. As a result, the sheet P2 is temporarily separated from the sheet P1.
Taking, the frictional force exerted between the circular-arch portion 40a of the sheet feed roller 40 and the sheet P1 as f1, the frictional force exerted between the sheet P2 and the separation pad 48 as f2, and the frictional force exerted between the sheets P1 and P2 as f3, the sheet feed rollers 40 and the separation pad 43 are arranged so as to satisfy the relation ship of f1&gt;f2&gt;f3. Therefore, even if both the sheets P1 and P2 are nipped between the circular-arch portion 40a of the sheet feed roller 40 and the separation pad 43 with the rotation of the sheet feed rollers 40, the sheet P2 is hindered from advancing by the frictional force exerted between the sheet P2 and the separation pad 43. As a result, the sheet P2 is separated from the sheet P1, and only the sheet P1 is fed. Since the separation pad 43 is on the course of rotation of the circular-arch portion 40a of the sheet feed roller 40, the separation pad 43 is pushed downward by the circular-arch portion 40a as a result of its rotation. However, since the movable idle roller 47 is forced toward the separation pad 43 by the spring 48, the separation pad 43 comes into contact with the movable idle roller upon depression, thereby resulting in the separation of the sheets.
The sheet feed rollers 40 perform exactly one rotation, and the sheet feeder returns to the standby condition (the state of the sheet feeder shown in FIG. 36).
Through the foregoing operations, only the uppermost sheet P1 is fed.
With such a sheet feeder, the sheet feed rollers 40 are prevented from being constantly held in pressed contact with the separation pad 43, and consequently the degree of abrasion of the sheet feed rollers and the separation pad is reduced.
Further, since the movable idle roller 47 is kept in pressed contact with the separation pad 43 by means of the spring 48, the next sheet is prevented from entering the nipping section together with the uppermost sheet to a certain extent.
&lt;Problem 1&gt;
In the mechanism shown in FIG. 35, since the lever 22 is pivoted by the urging force of the spring 26 when the next sheet P2 is reset. If the urging force of the spring 26 is small, a risk arises of failure to ensure reliable resetting of the next sheet P2. If the urging force of the spring 26 is increased, the next sheet S2 may be forcibly reset in a reliable manner. However, as shown in FIGS. 35C and 35D, the lever 22 must be pushed to the receded position by the sheet S1 at the time of feeding of the uppermost sheet S1. For this reason, an increase in the urging force of the spring 26 is not desirable. In order to set the lever 22 in such a way as to be pushed to the receded position by the uppermost sheet S1 in the case of the urging force of the spring 26 being increased, a force for feeding of the sheet S1, i.e., the nipping force exerted between the sheet feed rollers 18 and the separation pad 24, must be increased. In this case, the aforementioned original objective of reducing a load (or a back tension) cannot be achieved.
In order to reduce the load (or the back tension) by means of the foregoing existing mechanism, the urging force of the spring 26 must be reduced. If the urging force of the spring 26 is reduced, a risk arises of unreliable resetting of the next sheet S2.
&lt;Problem 2&gt;
In order to enable the previously-described sheet feeder; e.g., sheet feeder shown in FIG. 36, to appropriately feed various types of differently-sized sheet, or sheet having different widths, it is desirable to provide the sheet feeder with a pair of sheet feed rollers 40 and to make at least one of them slidable in accordance with the width of sheet.
Further, in order to enable sheet P to be readily loaded on the sheet mounting plate 42a while the sheet feeder is in a non-sheet-feeding state (or a standby condition), the sheet mounting plate 42 is preferably arranged so as to be separated from the sheet feed rollers 40 against the urging force of the spring 42c (or the sheet mounting plate 42 is moved downward) when the sheet feeder is in a non-sheet-feeding state.
With the foregoing arrangement, the sheet feeder shown in FIG. 36 presents the following problems.
For example, in a case where sheet having a comparatively wide width (e.g., A4-size sheet) is loaded on the sheet mounting plate 42a and sheet having a comparatively narrow width (e.g., B5-size sheet) is loaded in place of the A4-size sheet, the user sets the interval between the pair of sheet feed rollers to a narrow width in accordance with the width of the sheet in advance and loads sheet on the sheet mounting plate. There may be another case where sheet is initially loaded on the sheet mounting plate and the interval between the sheet feed rollers is adjusted to the width of the sheet.
In the case where the user loads sheet on the sheet mounting plate at the outset, the sheet mounting plate 42a still remains in the position separated from the sheet feed rollers 40 at the time of loading of sheet, as previously described. Because of this, the front edge portion of the top sheets (e.g., the sheets P1 and P2) of the loaded sheet P burrows into a clearance C formed between the sheet feed rollers 40 and the separation pad 43, as viewed in the axial direction of the sheet feed roller 40.
If an attempt is made to slide one of the sheet feed rollers 40 in accordance with the width of sheet, the front edge portion of the sheet burrowed into the clearance C comes into contact with the side surface of the sheet feed roller 40 or the side surface of the idle roller 47, or the like, thereby inhibiting smooth sliding action of the sheet feed roller 40. As a result, the sheet feed rollers 40 fail to slide to an optimum position. If the sheet feed rollers 40 are slid forcibly, the front edge portion of the sheet burrowed into the clearance C is nipped between the pair of sheet feed rollers 40 or by the idle roller 47, or the like, thereby rendering the sheet concertinated.
If the sheet is fed in while remaining in the concertinated state, sheet jams will occur, or the sheet will be fed diagonally.
Even if the mechanism shown in FIG. 35 is constructed so as to have the foregoing arrangement, i.e., a pair of sheet feed rollers at least one of which is slidable in accordance with the width of sheet and a sheet mounting plate which is separated from the sheet feed rollers when the sheet feeder is in a non-sheet-feeding state (or a standby condition), the previously-described problems seem not to arise, because the lever 22 is positioned so as to close the clearance formed between the sheet feed roller 18 and the separation pad 24.
However, in practice, another similar problem arises for the following reasons.
As is evident from FIG. 35E, the mechanism shown in FIG. 35 does not have any means for preventing the pivotal movement of the lever 22 in a counterclockwise direction (in a forward direction) in the drawings when the sheet feeder is in a non-sheet-feeding state.
The lever 22 is forced solely by means of the spring 26. Therefore, if sheet is loaded by a force that is sufficient to overcome the urging force of the spring 26, a risk arises of the front edge portion of the uppermost sheet burrowing into the clearance between the sheet feed roller 18 and the separation pad 24 while pivoting the lever 22, thereby imposing a problem analogous to the foregoing problems.
More specifically, if an attempt is made to construct the existing sheet feeder or sheet feeding mechanism in such a way as to enable at least one of a pair of sheet feed rollers to slide in accordance with the width of sheet and to separate the sheet mounting plate from the sheet feed rollers, a risk arises of the front edge portion of the sheet being loaded on the plate burrowing into the clearance formed between the sheet feed rollers and the separation pad when viewed in the axial direction of the sheet feed roller. For these reasons, sheet jams arise, or sheet is fed diagonally.
&lt;Problem 3&gt;
The previously described separation pad type sheet feeder is generally designed in such a way that the sheet is retained in a substantially horizontal state. For example, even in the case of the sheet feeders shown in FIGS. 35 and 36, the sheet is retained in a substantially horizontal state.
For this reason, the existing separation pad type sheet feeder requires a large footprint.
The problem can be solved by arranging the sheet feeder in such a way as to retain sheet in an inclined state. For example, if the sheet feeder is designed so as to retain the sheet at an angle of 45.degree., the footprint occupied by the sheet is reduced to half its original area.
However, it has been difficult to construct the separation pad type sheet feeder in such a way as to retain sheet in an inclined state.
For example, as is evident from the assumption that sheet would be retained in an inclined state (e.g. the sheet is inclined at about 45.degree.) in the sheet feeder shown in FIG. 35, it is possible that the sheet will slide under its own weight if the sheet is retained in an inclined state. As a result, the next sheet (S2), the sheet after the next (S3), or the like, cause an avalanche, thereby rendering the sheet considerably easy to burrow into the nipping section between the sheet feed rollers 18 and the separation pad 24. Particularly for the case of slippery sheet, e.g., sheets for OHP purposes, the sheet is apt to cause an avalanche, thereby rendering the sheet considerably easy to burrow into the nipping section between the sheet feed rollers 18 and the separation pad 24.
For these reasons, even if the lever 22 shown in FIG. 35 is provided for the sheet feeder, a risk may arise of the front edge of the next sheet (S2) having already passed through the front end of the lever 22 when the rear edge of the uppermost sheet (S1) passes the front end of the lever 22. As a result, a risk may arise of reliable separation of sheets becoming impossible.
Even if the sheet feeder shown in FIG. 36 is arranged so as to retain sheet in an inclined state, the next sheet or the sheet after the next may cause an avalanche in an analogous manner. Thereby rendering the sheet considerably easy to burrow into the nipping section between the sheet feed rollers 40 and the separation pad 43. Consequently, the next sheet gradually (or cumulatively) burrows into the nipping section between the sheet feed rollers and the separation pad every time the sheet feeding operation is performed, thereby rendering the separation of the sheet impossible.
It is conceivable that such a problem can be solved by considerably increasing the nipping force exerted on the sheet feed rollers 18 and the separation pad 24 (or the nipping force exerted on the sheet feed rollers 40 and the separation pad 43 and the nipping force exerted on the elastic idle roller 47 and the separation pad 43 in the sheet feeder shown in FIG. 36).
However, if the nipping force is increased, the drive force exerted on the sheet feed rollers must also be increased. Since the previously-described load (or back tension) which is applied to the sheet by the conveyor rollers 2, 3 is considerably increased, the nipping force applied to the sheet by the conveyor rollers 2, 3 must be increased so as to ensure a feed force which is sufficient to overcome the load. Accordingly, a large drive force to drive the conveyor rollers, or the like, is also required, thereby rendering the sheet feeder bulky or resulting in an increase in power consumption. Further, the sheet rollers or the conveyor rollers become more susceptible to abrasion.
In short, it has been impossible to retain sheet in an inclined state and to feed the sheets of sheet one by one without increasing the drive force of various rollers or the back tension even by means of the technique shown in FIG. 35 and the technique shown in FIG. 36.
&lt;Problem 4&gt;
In the event of a plurality of sheets concurrently burrowing into the clearance between the sheet feed rollers and the separation pad or the clearance between the idle roller and the separation pad as a result of occurrence of an avalanche in any one of the previously-described existing sheet feeders, the sheets are integrated into the form of a wedge, thereby resulting in a risk of the uppermost sheet being locked during the course of sheet feeding operation.